This invention relates to a system and a method for converting particulate matter in exhaust gases from stoichiometrically-operated gasoline engines.
Gasoline engines are in widespread use in all types of vehicles and stationary power sources. Because of their combustion characteristics they generate particulate matter (PM) that can be a solid, fine droplets of liquid or a mixture of the two. The PM may comprise a variety of products from the combustion event, such as unburned hydrocarbons (HC) and sulfuric acid produced by the oxidation of sulfur dioxide derived from sulfur species present in the fuel. Although less visible to the naked eye than PM produced by diesel engines, PM generated by gasoline engines is potentially more harmful to humans. Due to the small particle size, PM penetrates deep into alveolar passages within the lung. This has been linked with causing lung cancer. Diesel exhaust gases also contain other components which need to be treated to meet emission legislation, including NOx.
In order to meet emission legislation, it has become commonplace to fit gasoline vehicles with a catalytic converter called a three-way catalyst (TWC). The most common TWCs use a platinum group metal (PGM), such as one or more of Pt, Pd and Rh, carried on an oxidic, high surface area washcoat, which can be one or more of Al2O3, CeO2, ZrO2 or mixtures of any two or more thereof. The support for the catalyst is generally a low pressure-drop flow-through honeycomb monolith, manufactured from an extended ceramic e.g. cordierite, or from a metal. However, the TWC achieves only partial removal of PM.
The removal of PM can also be achieved using some form of filter or trap, which may be cleaned or regenerated intermittently or continuously. A particulate trap may itself be catalyzed to lower the PM combustion temperature and some form of external heating, for example electric heating of the trap or of the air fed thereto may be used to initiate PM combustion. It has also been suggested to include a catalyst in the fuel to the engine, and, as well as PGMs, iron, copper or cerium compounds have been suggested as potential catalytic materials.
However, the application of these prior art suggestions to stoichiometrically-operated engines are associated with one or more problems including inadequate removal of PM compared with control of NOx, excessive cost, reduced fuel economy and incompatibility with existing technology.
A successful particulate trap for use in diesel applications is marketed by Johnson Matthey Plc as the xe2x80x9cContinuously Regenerating Trapxe2x80x9d (CRT(trademark)) and is described in U.S. Pat. No. 4,902,487. The system uses NO2 derived from oxidizing NOx in the exhaust gas to combust PM disposed on a filter. This is advantageous for diesel applications since NO2 can combust PM at about 250xc2x0 C., and the temperature of diesel exhaust is generally up to 350xc2x0 C. However, a temperature of up to about 650xc2x0 C. is required to combust PM in O2.
The use of the Continuously Regenerating Trap is generally limited to diesel applications because typically there is not enough oxygen present in the exhaust from gasoline engines to effect the oxidation of NO to NO2 as any O2 present reacts preferentially with other exhaust gas components such as CO, hydrogen, and hydrocarbons. Gasoline engines, other than gasoline direct injections (GDI) engines, normally function under stoichiometric or slightly rich conditions.
By xe2x80x9cstoichiometricxe2x80x9d is meant the point at which there is theoretically enough oxygen to consume all of the hydrocarbons and carbon monoxide in the combustion event. The point occurs at an air to fuel ratio of about 14.7. Under stoichiometric conditions, the exhaust gas is neither reducing nor oxidizing in nature. An exhaust gas can be described as xe2x80x9crichxe2x80x9d when it is reducing in nature owing to the presence of fuel residues from the combustion event i.e. the air to fuel ratio at combustion was less than that at the stoichiometric point. xe2x80x9cLeanxe2x80x9d exhaust gas conditions result when the air to fuel ratio during combustion is greater than at the stoichiometric point. Lean exhaust gases include excess oxygen and little or no hydrocarbons because of near complete combustion of the hydrocarbon fuel and the exhaust gases will be oxidizing in nature as a result. In practice, the exhaust gases of an engine set up to run at the stoichiometric point will vary slightly in composition because of macro-fluctuations about the neutral non-oxidizing/non-reducing norm of stoichiometry e.g. when the engine is idling (slightly lean) or being used to accelerate a vehicle (slightly rich). But on average, the exhaust gases are non-oxidizing and non-reducing. For ease of description the use of the term xe2x80x9cstoichiometricxe2x80x9d or xe2x80x9cstoichiometricallyxe2x80x9d hereinafter embraces conditions in which there are macro-fluctuations around the true stoichiometric air to fuel ratio, and the composition of the exhaust gases varies accordingly.
WO 99/18333 describes a method and apparatus for using free radicals generated by action of a plasma generator on water vapor in an exhaust gas to enhance the action of a TWC on the oxidation of unburned HC and CO and the reduction of NOx.
U.S. Pat. No. 5,746,984 describes an exhaust system for use in combination with a lean-burn engine such as a gasoline direct injection engine including a NOx trap, an HC trap, a particulate trap and a plasma generator. In the system, trapped NOx and HC components are released under rich conditions and are carried to the plasma generator where the unburned HC, NOx and PM are destroyed using plasma.
Our WO 00/21646 describes a system for the purification of exhaust gases in diesel engines, which system includes a plasma generator and a filter or particulate trap. This system is for treating exhaust gases from diesel and other lean-burn engines.
We are not aware of any efforts having been made specifically to remove PM from exhaust gases of stoichiometrically-operated engines.
We have now discovered that, by treating water vapor present in exhaust gases from a stoichiometrically-operated gasoline engine with a plasma, the plasma-treated water vapor can be used to convert PM, which can be disposed on a filter, and/or to oxidize NO to NO2. The NO2 is then used to combust the PM.